This invention relates to the recovery of molybdenite from mixed sulfide ores, particularly those of iron and copper, contained in flotation concentrates.
Molybdenum, which occurs in nature as its sulfide ore molybdenite, MoS.sub.2, is usually found in association with other metallic sulfides, chiefly those of copper and iron. Molybdenite is usually present as a minor component of such mixed sulfides ores. Molybdenite is commonly recovered as a byproduct of the copper refining process. In the copper refining process the metallic sulfide contained in the ores are concentrated by bulk flotation by employing differential wetting of the mineral surface. Differential wetting of the mineral surface is induced by addition of specific organic collector reagents to aqueous slurries of the finely ground ore.
The first step in the recovery of molybdenite is that of bulk flotation during which the copper and molybdenum sulfides are gathered together in a flotation concentrate which, in the copper refining process, is commonly termed a copper concentrate. The copper concentrate contains the majority of the copper and molybdenum content of the original ores plus some iron sulfides and some portion of the original host rock, usually referred as gangue. The composition of a copper concentrate may vary greatly, but in typical operations such concentrates contain from about 10 to about 35 percent by weight copper, and from about 0.2 to about 5 percent by weight molybdenum.
While many processes exist for separation of molybdenite from a copper concentrate, those processes which are most widely used generally consist of three sequential steps. The first step comprises removing the organic collector reagents which were added during the bulk flotation process to render the metallic sulfides hydrophobic. In the second step the stripped copper concentrate is contacted with a reagent to render the copper and iron sulfides hydrophilic relative to the molybdenite. Thereafter, in the third step, molybdenite is differentially floated from the copper-iron sulfides by a process of multi-stage froth flotation and the molybdenite is recovered as an enriched concentrate.
Several procedures, physical and chemical, exist for stripping copper concentrates of organic collectors. Two physical processes which are used are roasting and streaming. Roasting has the drawback that the molybdenite may be oxidized and thus also rendered hydrophilic. In U.S. Pat. No. 2,255,776 various heat treatment processes, including steaming, are described for stripping copper concentrates. U.S. Patent application Ser. No. 900,830 describes the stripping of organic collectors by steaming in the presence of activated carbon. Chemical processes used to strip the copper concentrate of organic collector reagents include leaching under acidic conditions, as described in U.S. Pat. No. 2,664,199, or the addition of oxidizing agents to the copper concentrate as described in U.S. Pat. No. 2,559,104. The most commonly used oxidizing agent is hydrogen peroxide, which may be used alone, as described in U.S. Pat. No. 3,137,649, or in the presence of soluble metal salts, as described in U.S. Pat. No., 3,811,569.
After stripping, the stripped concentrate is contacted with a reagent, usually referred to as a copper depressant, which renders the copper sulfides hydrophilic relative to the molybdenite. Several reagents are currently used, either alone or in combination. But each of the conventionally used depressants have a number of disadvantages. The most common depressant reagents are sulfides, or hydrosulfides of ammonia or alkali metals, particularly sodium hydrosulfide and ammonium sulfide. These widely used reagents are effective copper depressants, but they are susceptible to oxidation which destroys their activity. As a result, such reagents must be used in large quantities to be effective, typically from about 5 to about 35 pounds per ton of copper concentrate. The effectiveness of the hydrosulfide ion can be improved by employing flotation conditions which avoids its oxidation. An effective means to accomplish this is to substitute an inert gas, such as nitrogen as described in Canadian Pat. No. 1,070,034, for the air normally used in flotation. However, use of inert gases requires the addition of a specialized plant to produce the large volume of inert gas required, hence increases the cost of molybdenite recovery.
U.S. Pat. No. 2,664,199 describes the use of sodium ferrocyanide as a copper depressant. But sodium ferrocyanide is only effective when added to copper concentrates in which the predominant copper mineral is chalcocite, Cu.sub.2 S. It is less effective when used to treat cahlcopyrite, CuFeS.sub.2. Because of this, sodium ferricyanide is sometimes used with sodium ferrocyanide or alone or chalcopyrite. Nokes' reagent, which is the unisolated product of the reaction between P.sub.2 S.sub.5 and NaOH, as described in U.S. Pat. No. 3,375,924, is another copper depressant presently in use. However, Nokes reagent is not widely used because of the continual need to prepare the reagent, its potential safety hazard in preparation and the relatively high cost of its ingredients. Cyanide ion, in the form of one of its soluble salts, is also used as a depressant for copper and iron. But, its extreme toxicity to man and aquatic life limits its use. Several organic reagents have been proposed, or find limited use as copper depressants. One is thioglycolic acid, while U.S. Pat. No. 3,329,266 describes the use of the xanthate formed by the reaction of carbon disulfide and hydroxy acetic acid. Organic reagents are generally ineffective when used alone, and usually require the addition of another depressant such as cyanide. Further, for such reagents to be effective as copper depressants the copper concentrate may require stripping of the collectors before addition of the reagent.
Of the three step sequence discussed above, that step which is most important to obtaining separation of molybdenite from a copper concentrate is the step of adding a copper depressant reagent. The separation of molybdenite is very tedious unless a reagent is added to render the copper sulfides hydrophilic.